Two widely used methods for identifying objects today are barcodes and magnetic strips. Bar codes are commonly used for identifying objects in shops and supermarkets. An application of magnetic strips is the credit card. The main reason for the popularity of barcodes and magnetic strips is that they are inexpensive. One drawback of barcodes and magnetic strips is the distance range in which they can be used. The reader has to have a physical contact or has to be very close, say a few centimetres. If there is no physical contact, then the space between the code and the reader should not have any obstruction. In addition, the reader and the code have to be properly aligned for correct readability. This demands concentration from the part of the human operator and therefore is inconvenient.
RFID (radio frequency identification) tag (or RFID-tag) is another technology used for identifying the identity of an object. In an RFID system, the interrogator or the reader and the tag can be separated by a larger distance compared to that of the magnetic strip technology or the bar-code technology. Once interrogated by the reader, RFID-tags will return an encoded radio signal that contains the identity of the object. RFID-tag devices can be broadly divided based on the criteria weather they contain an integrated memory chip or not. Those that contain a memory chip, e.g. U.S. Pat. No. 5,874,902, in general have more memory capacity than those of chip-less tags, e.g. U.S. Pat. No. 6,708,881. However, chip based tags have a significantly higher cost compared to that of the chip-less tags. RFID-tags can also be divided based on the criteria weather they contain a battery or not, active and passive tags. In general active tags, which are the most commonly available tags in market today, have a larger operational distance range when compared to the passive tags, e.g. U.S. Pat. No. 6,621,417.